The present invention relates to an attachment for a sewing machine, and more particularly to an overcasting attachment for use with a sewing machine which is prepared for straight stitching, wherein needle-like thread guide pieces are provided for passing an additionally prepared thread or threads from sideways around the needle point at a suitable time, making use of the vertical reciprocating movement of the needle, so as to engage the thread or threads with the sewing machine threads for straight stitching.